The 5' ends of all trypanosome mRNAs have recently been shown to have a common 35 nucleotide spliced leader (SL). The SL is believed to arise by transcription within a miniexonderived RNA (medRNA) of about 140 nucleotides, followed by its transsplicing with the primary transcripts of mRNAs. The presence of the SL in all trypanosomal mRNAs provides an ideal system to rest the efficacy of short oligonucleotides as therapeutic agents. Synthetic oligonucleotides, consisting of diesters, ethyl phosphotriesters or methylphosphonates, will be synthesized. These will be complementary to the SL sequence or to the splice donor site of the medRNA in order to directly inhibit protein synthesis or mRNA maturation. These oligonucleotides will be tested for their ability to inhibit protein synthesis in cell free systems using purified mRNA, and in cell cultures of Trypanosoma brucei. Specific, antisense oligonucleotides will provide valuable chemotherapeutic agents for the treatment of important human diseases.